Conventionally, in a hybrid vehicle having an engine and a generator motor coupled to each other as the drive sources, a capacitor may be used as a power storage device for exchanging electric power with the generator motor. Voltage of the capacitor gradually decreases due to continuous operation. Therefore, when the voltage of the capacitor becomes lower than operable voltage of the capacitor, the system is rendered inoperative.
As one method of solving the above-described problem to prevent the system from being inoperative, capacitance of the capacitor may be increased. However, when the capacitance of the capacitance is increased, the capacitor size is increased and there may be a problem of weight, a mounting space, and a cost thereof might occur.
Then, as a technique to prevent the system from being rendered inoperative without increasing the capacitance of the capacitor, a technique to limit an output of a voltage converter for converting electric power of the capacitor to electric power having low potential when a power storage amount of the capacitor becomes lower than a predetermined threshold during engine stop is disclosed (for example, refer to Patent Document 1).    Patent Document 1: Japanese Laid-Open Patent Application Publication No. 2005-45883